The black water, heavy with debris, came tumbling apparently out of nowhere, gushing over the rocky terrain and pushing boulders around like toys.

This torrent is known as a glacial outburst flood, and it forms when water stored deep within a glacier is released without warning. A team of scientists witnessed this rare event firsthand as one spilled down the Lhotse Glacier near Mount Everest on 12 June 2016.

Their view, captured on video, affords researchers and the public an up-close look at the mechanics of a glacial outburst flood. “All of our studying was suddenly brought into focus, and revised, as we witnessed this event,” said Elizabeth Byers, a hydrologist at the consulting firm Appalachian Ecology in West Virginia and a member of the team. The team published their scientific findings and video in a communication brief last month in The Cryosphere.

“Glacial outburst floods are widespread, but observations of them are rare, especially by scientists,” said Mark Carey, a glacier researcher at the University of Oregon not involved in the study. “This firsthand assessment illuminates more details about how floods work.”

Racing Downhill

In 2016, a team of American researchers was working in Nepal near Imja Lake, one of the region’s largest glacial lakes. They were collaborating with local communities to improve awareness of glacial lake outburst floods, a perennial danger for those downstream of Imja Lake and its notoriously unstable moraine dam. On the morning of 12 June, some of the scientists were doing fieldwork near Lhotse Glacier, an avalanche-fed glacier that originates at the peak of Lhotse, the fourth-highest mountain in the world.

Byers remembers hearing what sounded like a rock fall. She then saw a “black tongue of water, boulders, and silt” racing downhill toward the village of Chukhung. She grabbed her camera and shot the video.

“I felt powerless to help people…and at the same time experienced utter fascination at the geologic process unfolding before my eyes.”“I have never experienced such adrenaline,” Byers said. “I felt powerless to help people…and at the same time experienced utter fascination at the geologic process unfolding before my eyes.”

The glacial outburst flood that Byers and her colleagues witnessed slowly subsided over the next hour. But the trail below the researchers had been washed away, and the only route to Chukhung was over the glacier itself, crossing an ice bridge with water still moving rapidly below it, Byers recalled.

The scientists raced over the ice bridge and down into Chukhung. They found that all of the villagers were accounted for and that the community had lost only one outhouse. The large rock wall that the community had built 1 year ago—using donations from the scientists after the 2015 earthquake—had held. “It was twisted in some places but enough to keep the floodwaters from destroying the village,” said Byers.

The scientists later estimated that more than 2 million cubic meters of water—enough to fill the Empire State Building twice—had been released in the flood.

Hidden Deep Within

Although the scientists were in Nepal to better understand floods from glacial lakes, the glacial outburst flood—in which the water spilled not from a lake but from within the glacier itself—provided them with an important opportunity to examine flow patterns from sudden events. Beginning on 14 June, the researchers returned to Lhotse Glacier to do fieldwork and reconstruct the path of the floodwater.

“A significant amount of the floodwater was stored in the glacier’s subsurface.”The scientists measured and photographed the terrain and found sinkholes, ponds, uprooted alpine shrubs, and wet sediments, all signs of recent flooding. They also examined satellite imagery of Lhotse Glacier taken in May and noted 274 ponds atop the ice. By measuring the ponds’ areas in the images and using a known relation between pond area and volume, the scientists estimated the total volume of all of the visible bodies of water.

Assuming that all of the ponds had drained completely, their total volume would be only about 20% of the estimated flood volume, the researchers calculated. Thus, “a significant amount of the floodwater was stored in the glacier’s subsurface,” the authors wrote.

Swiss Cheese, but Ice

Within a glacier, water travels in conduits, which can be as large as several meters across. Swiss cheese is “a very good analogy” to explain a glacier’s interior structure, said David Rounce, a glaciologist at the University of Texas at Austin and a member of the research team.

Sometimes those conduits can become blocked by ice or debris such as sand and boulders. When sufficient water pressure builds up behind the natural dam to cause it to burst, the torrent that’s released can be substantial, like the flooding that the team observed in June.

“These floods are particularly difficult to prepare for because there is often no visual evidence of their threat.”“These floods are particularly difficult to prepare for because there is often no visual evidence of their threat,” said Duncan Quincey, a glaciologist at the University of Leeds who reviewed the paper.

This April, the researchers will be returning to Nepal to study Imja Glacier. No one can predict when another glacial outburst flood might strike, but team members will have their cameras ready. “It’s possible that we’ll witness another one,” said Rounce.

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